(a) Field of the Invention
The present invention relates to a fuel cell system, and more particularly, to a stack of a fuel cell system and a cooling structure of the stack.
(b) Description of the Related Art
In general, a fuel cell is an electricity generating system that converts chemical energy into electric energy through a chemical reaction between oxygen contained in air and hydrogen contained in hydrocarbon-containing materials such as methanol, ethanol, and natural gas. Certain fuel cells also generate heat as a byproduct of the chemical reaction, which can be simultaneously used with the electrical energy.
Such fuel cells are classified into categories including a phosphate fuel cell working at a temperature of about 150° C. to 200° C., a molten carbonate fuel cell working at a high temperature of about 600° C. to 700° C., a solid oxide fuel cell working at a high temperature of 1,000° C. or more, and a polymer electrolyte membrane fuel cell and an alkali fuel cell working at a room temperature or at a temperature of 100° C. or less, depending upon kinds of used electrolyte. All of the fuel cells work on basically the same principle, but are different from one another in type of fuel, operating temperature, catalyst, and electrolyte.
The recently developed polymer electrolyte membrane fuel cell (PEMFC) has an excellent output characteristic, a lower operating temperature, and fast starting and response characteristics compared with other fuel cells. It uses hydrogen obtained by reforming methanol, ethanol, natural gas, etc. as fuel. Accordingly, the PEMFC has a wide range of applications such as a mobile power source for vehicles, a distributed power source for homes or buildings, and a small-sized power source for electronic apparatuses.
The aforementioned polymer electrolyte membrane fuel cell requires a fuel cell main body called a stack, a fuel tank, and a fuel pump that supplies fuel to the stack from the fuel tank. The polymer electrolyte membrane fuel cell further comprises a reformer that converts the fuel from the fuel tank to generate hydrogen gas and then supplies the hydrogen gas to the stack. The fuel stored in the fuel tank is supplied to the reformer by means of the fuel pump. Then, the reformer converts the fuel and generates the hydrogen gas. The hydrogen gas and oxygen with each other in the stack, thereby generating electric energy.
Alternatively, such a fuel cell may employ a direct methanol fuel cell (DMFC) scheme which directly supplies liquid-state methanol fuel to the stack. The DMFC fuel cell does not require the reformer, unlike the polymer electrolyte membrane fuel cell.
In the fuel cell system described above, the stack has a tower structure of several or several tens of electricity generators, each generator having a membrane-electrode assembly (MEA) and separators (or bipolar plates). The membrane-electrode assembly includes an anode (also referred to as “fuel electrode” or “oxidation electrode”) and a cathode (also referred to as “air electrode” or “reduction electrode”) that are attached to each other with an electrolyte membrane interposed therebetween. The separator simultaneously functions as a passageway through which oxygen and hydrogen gas required for the reaction of the fuel cell are supplied and as a conductor connecting the anode and the cathode of each membrane-electrode assembly in series. Fuel gas containing hydrogen is supplied to the anode and oxygen gas containing oxygen is supplied to the cathode through the separator. Through this interaction, an oxidation reaction of the fuel gas takes place in the anode and a reduction reaction of the oxygen gas takes place in the cathode. The movement of electrons generated by the reactions also results in heat and water byproducts.
The stack should be maintained at a suitable temperature in such a fuel cell system in order to stabilize the electrolyte membrane and to prevent deterioration in performance. To moderate the temperature of the stack, a conventional fuel cell system employs a typical air cooling system in which the heat generated from the stack during operation is cooled by air having a relatively low temperature, or a water cooling system in which cooling water is supplied to the stack to cool the heat generated from the stack.
The water cooling system requires an additional cooling plate for passing the cooling is water into the stack which makes it difficult to decrease the whole size of the fuel cell system.
In a conventional fuel cell system, when unreacted air containing moisture flows from the stack and is exhausted into an atmosphere having a relatively lower temperature, the moisture condenses as it comes into contact with the atmosphere. An additional unit for storing or recycling the water generated through the condensation of the unreacted air must be provided, but that would make it more difficult to decrease the whole size of the fuel cell system. In addition, the thermal or electrical load for driving the additional member further deteriorates the efficiency and performance of the whole fuel cell system.
Furthermore, the conventional fuel cell system heats liquid-state fuel required for generation of electricity and generating hydrogen gas with the reformer. Since the thermal load is further increased by this heating of the fuel, the efficiency and performance of the whole fuel cell system is deteriorated.